Fixed urllib3 warning about use of insecure connections

When using a PlantUML server with self-signed certificates it is necessary to set the insecure configuration option to true, allowing for insecure connections.
But in this case 'urllib3' gives us a warning that it will appear on the rendered page; this release resolves this issue.
It also solves another small problem, when using other types of diagrams (such as plantgantt) with a PlantUML server.